1. Field of the Invention
The present invention relates to an optical fiber comprising at least one epoxidized polyolefin based coating layer, and to a crosslinkable composition which can be applied as said coating.
More particularly, the present invention relates to an optical fiber comprising at least one epoxidized polyolefin based primary coating layer and at least one secondary coating layer deposited around said primary coating, and to a crosslinkable composition which can be applied as said primary coating.
2. Description of the Related Art
Optical fibers commonly consist of a glass portion (typically with a diameter of about 125 μm), inside which the transmitted optical signal is confined, and of a coating, typically polymeric, arranged around the glass portion for substantially protective purposes. This protective coating typically comprises a first coating layer positioned directly onto the glass surface, known as the “primary coating” or “primary” for short, typically having a thickness of between about 25 μm and about 35 μm. In turn, this primary coating is generally covered with a second coating layer, known as the “secondary coating” or “secondary” for short, typically having a thickness of between about 10 μm and about 30 μm.
These polymer coatings may be obtained from compositions comprising oligomers and monomers that are generally crosslinked by means of UV irradiation in the presence of a suitable photo-initiator. The two coatings described above differ, inter alia, in terms of the modulus of elasticity of the crosslinked material. As a matter of fact, whereas the material which forms the primary coating is a relatively soft material, with a relatively low modulus of elasticity at room temperature, the material which forms the secondary coating is relatively harder, having higher modulus of elasticity values at room temperature. The combination of said two layers of coating ensures adequate mechanical protection for the optical fiber.
The optical fiber thus composed usually has a total diameter of about 250 μm. However, for particular applications, this total diameter may also be smaller; in this case, a coating of reduced thickness is generally applied.
The crosslinking of the abovementioned compositions, depending on the reactive groups present in the compounds (oligomers and monomers) to be crosslinked, may take place, for example, via a free-radical or cationic route. Typically, the crosslinking of compounds comprising epoxide groups takes place cationically.
For example, patent application EP 124 057 describes a cationically crosslinkable liquid composition comprising a polyepoxide, a polysiloxane bearing a plurality of hydroxyalkyl groups in the molecule, and a photo-initiator and/or a photo-sensitizer. According to the assertions made in the application, said composition is capable of providing a coating for optical fibers that is capable of maintaining low modulus of elasticity values at low temperatures (−60° C.) so as to avoid the phenomenon known as “microbending”, with consequent attenuation of the transmitted signal.
Patent application EP 533 397 describes an optical fiber with a coating which includes at least one layer comprising a cationically crosslinkable composition. Said composition comprises a resin containing cationically crosslinkable end groups, a diluent containing cationically crosslinkable end groups, and a photo-initiator. Resins that are useful for this purpose may be selected from vinyl ether resins and epoxy resins. According to the assertions made in the application, said cationic crosslinking leads to the formation of an acid medium in contact with the glass portion of the optical fiber, making it possible to obtain an optical fiber with improved mechanical strength.
Patent U.S. Pat. No. 6,042,943 describes a radiation-crosslinkable composition which may be used as primary coating for an optical fiber, comprising: (a) a compound comprising (i) a saturated aliphatic chain, and (ii) at least one epoxide group at one end and at least one reactive function, which may be selected from acrylates, vinyl ethers, hydroxyls, or combinations thereof, at the other end; (b) a blend of acrylate-type monomers comprising (iii) a first monomer containing an acrylate group, and (iv) a second monomer containing at least two acrylate groups; and (c) a photo-initiator. According to a further embodiment, (b) is a blend of monomers comprising (iii) a first monomer containing an acrylate group or a vinyl ether group, and (iv) a second monomer containing at least two functional groups which may be selected from acrylates, epoxides, vinyl ethers and hydroxyls. According to a further embodiment, (b) is a blend of monomers of vinyl ether type comprising (iii) a first monomer containing a vinyl ether group, and (iv) a second monomer containing at least two vinyl ether groups. According to a further embodiment, (b) is a monoacrylate residue containing from 6 to 20 carbon atoms.
Patent U.S. Pat. No. 5,993,965 describes a fiber with a coating based on a hydrophobic material derived from the photo-polymerization of a composition comprising at least one epoxidized polydiene oligomer, at least one photo-initiator and, optionally, a reactive diluent of monomeric type. According to the assertions made in the patent, the fibers thus coated are said to have improved mechanical behaviour. In particular, said coating is used in optical fibers.
As observed by the Applicant, the use of reactive diluents of monomeric type, that are generally required to obtain compositions whose viscosity allows them to be applied to optical fibers at room temperature, may present some drawbacks. For example, the relatively low molecular weight of these monomer components is connected with a relatively high volatility, with a consequent contamination of other materials and/or risks to the environment and to the health of the workers. In addition, following the crosslinking by UV irradiation, residues of unreacted components may remain in the final resin. The presence of these unreacted monomer residues within the polymer network may result in unwanted phenomena of extraction by water and/or waterblocking fillers commonly used in optical cables to prevent or limit the entry of water into the structure of the cable. This extraction entails a worsening in the mechanical properties and may also result in the initiation of the phenomenon of delamination of the fiber, i.e. detachment of the polymer coating from the glass portion of the fiber, with possible generation of the phenomenon known as “microbending”.